1) Field of the Invention
The invention relates to pore-free sintered bodies on the basis of silicon carbide which are suitable as substrates for hard disks in magnetic disk drives, and to methods for the fabrication thereof.
2) Background Art
Materials suitable as substrates for hard disks must be amenable to extremely high-quality polishing, with a high flatness and low porosity (IDEMA specifications) and a roughness "Ra" below 100 .ANG., preferably below 10 .ANG.. They must be very stiff so that, in the usual form of thin disks (for example 65.O slashed..times.20.O slashed..times.0.635 mm), they will not distort when mounted in the drive or under their own weight or when being heated to about 300-600.degree. C. while the magnetic layers are being sputtered on or when they rotate in the drive at e.g. 7500 rpm. Otherwise, a uniform distance of the read-write head would not be ensured and reading and writing of data in the magnetic layer would give rise to errors. Moreover there would be the risk of the disk colliding with the head and possibly being destroyed.
Furthermore, the substrate material is required to be very hard so that an occasional collision with the read-write head will not result in deformations in the substrate and loss of data.
As a material, SiC is distinctly superior, in all these points, to the substrates already being used:
a) aluminum with Ni/P coating PA1 b) glass or glass ceramics. PA1 a) from 45 to 99.5 wt % of SiC, the SiC being present in the sintered body as a crystalline first phase, and PA1 b) from 0.5 to 55 wt % of a sintering aid, from 0.5 to 30 wt % of which is selected from the group consisting of reaction products of Al.sub.2 O.sub.3 with Y.sub.2 O.sub.3, mixtures of at least one nitrogen-containing aluminum compound with reaction products of Al.sub.2 O.sub.3 with Y.sub.2 O.sub.3, mixtures of at least one rare earth oxide with at least one nitrogen-containing aluminum compound and/or Al.sub.2 O.sub.3, and from 0 to 25 wt % of which is selected from the group of the nitridic silicon compounds, the sintering aid being present in the sintered body as a second phase and optionally further phases, wherein the second phase and optionally the further phases are either amorphous to an extent of more than 10% or are present in amorphous form at the interface to the first phase to a width of at least .gtoreq.5 .ANG. and where the polished surface of the sintered body does not contain any pores having a diameter .gtoreq.1 .mu.m.
Its modulus of elasticity, as a measure for its stiffness, is about 400 Gpa and thus about 5 times the modulus of elasticity of a) or b) and its hardness (HV 2500) is likewise about 4 times as high as that of a) or b).
In addition, its thermal stability allows higher sputtering temperatures, resulting in potentially superior magnetic layers. Thus, the sputtering temperature in the case of a) is limited to .ltoreq.300.degree. C., because the material will otherwise become magnetic. In the case of glass, with the chemical hardening customary for this application, the sputtering temperature is limited to about 400.degree. C. because above this temperature, ion diffusion will result in the hardness effect being lost and there is a risk of deformation.
SiC, in contrast, remains unchanged even at higher sputtering temperatures aimed for, e.g. 500.degree. C.
On a long-term basis, SiC therefore provides for thinner and consequently lighter disks (energy consumption/volumetric data density), higher rotational speeds (more rapid access to data) and lower flying heights (more bits/cm.sup.2) than Al or glass or glass ceramics.
Moreover, for novel substrates for hard disks to be able to compete with the known materials, it ought to be possible to produce them at a low price.
One of the first publications in which SiC is proposed for hard disks is U.S. Pat. No. 4,738,885 (Kyocera) which issued Apr. 19, 1988. The patent states that ceramics sintered at atmospheric pressure are usually highly porous and that hot isostatic pressing is necessary after sintering to close the pores.
U.S. Pat. No. 5,465,184 (CVD Inc.) issued Nov. 7, 1995 and U.S. Pat. No. 5,474,613 (CVD Inc.) issued Dec. 12, 1995 disclose pore-free free-standing CVD-SiC substrates for use as hard disks. Referring to sintered SiC, this publication states that the said material always contains pores.
U.S. Pat. No. 5,480,695 (Tenhover) which issued Jan. 2, 1996 exhaustively discusses the prior art in the field of ceramic substrates for hard disks and discloses a sintered SiC hard disk substrate whose porosity is covered over by a dense, nonstoichiometric SiC layer sputtered onto it. Regarding the sintered material SiC, it is stated that this material is inherently porous and without a coating is therefore not suitable for the fabrication of hard disks.
U.S. Pat. No. 5,302,561 which issued Apr. 12, 1994 and U.S. Pat. No. 5,358,685 which issued Oct. 25, 1994 (Cercom) disclose a special hot-pressed SiC and the corresponding production process. The description states said SiC to be free of pores and that no grains are polished out during polishing and that it is therefore suitable for use as a hard disk.
The only materials known in the prior art which are suitable for the fabrication of hard disks are therefore SiC sintered under pressure (via hot pressing or hot isostatic pressing) or CVD SiC deposited from the gas phase. SiC sintered at atmospheric pressure in this context, given its porosity, requires a coating. So far, all these processes have failed, on technical grounds and on the grounds of cost, to become generally accepted.
It is therefore desirable for materials on the basis of silicon carbide for the fabrication of hard disks to be available which can be produced by means of cost-effective sintering methods.